Method of operating a welding power supply and a welding power supply

ABSTRACT

A method of operating a welding power supply ( 20 ) during a welding process in which an electric arc ( 95 ) between a consumable electrode ( 94 ) and a work piece ( 102 ) is generated while feeding the consumable electrode ( 94 ) and moving the arc ( 95 ) in relation to the work piece ( 102 ) along a welding track ( 103 ), wherein a transition between a DC power output of the welding power supply and an AC power output of the welding power supply, or vice versa, is made without interruption of the welding process.

TECHNICAL FIELD

The present invention relates to a method of operating a welding powersupply. In particular the invention relates to a method of operating awelding power supply which may be set to generate a DC power output ofthe welding power supply as well as an AC power output from the sameoutput terminal. The invention furthermore relates to a welding powersupply which is designed to generate a DC power output of the weldingpower supply as well as an AC power output from the same outputterminal.

BACKGROUND

In welding technology a diversity of welding processes are present.These processes include for example tungsten inert gas welding (TIG),MIG/MAG and submerged arc welding (SAW). In TIG technology an arc isgenerated between a non-consumable electrode and the work piece. Ifdesired a metal filler is fed into the arc. The TIG technology issuitable for welding in thin materials, in particular welding of thinaluminum work pieces. In MIG/MAG and submerged arc welding, an arc isgenerated between a consumable electrode and a work piece. MIG/MAG issuitable for welding of all kinds of metals at medium thickness. In TIGand MIG/MAG welding a weld puddle generated by the arc is protected by agas supplied from a shield cup arranged at a welding torch. In submergedarc welding (SAW) an arc is generated between a consumable electrode anda work piece under a protective layer of flux covering the work piece atthe arc. MIG/MAG is suitable for welding of all kinds of metals wherehigh deposition rates are required, such as when welding in thickmaterials.

In the field of welding different parameters may be adjusted to achievea desired result. These parameters includes welding voltage, weldingcurrent, electrode feed speed and welding propagation speed.

Furthermore, welding processes may be performed by a direct currentprocess with an output from the power source connecting the electrode tothe negative potential, a direct current process with an output from thepower source connecting the electrode to the positive potential or as analternating current process where the electrode switches betweenelectrode negative and electrode positive. Generally electrode negativeprovides for a wide weld bead with low penetration and electrodepositive provides for a a narrow bead with deep penetration. Thealternating current process can be seen as a process having propertiesin between the DC-negative and DC-positive process. Generally thealternating current has a base frequency of around the net frequency.Optionally, the frequency can be higher, that is the region of 200-400Hz. High frequencies will generate losses in welding cables and istherefore not suitable for many applications.

Welding power sources that may operate in either DC mode or AC mode arepreviously known. One example is disclosed in U.S. Pat. No. 4,517,439where separate AC and DC terminals are provided.

Even though the prior art is rich when it concerns improvements incontrol of the welding power supplies to generate weld seams with highquality, it is desirable to provide further improved methods foroperating welding power supplies.

It is thus an object of the present invention to provide an improvedmethod of operating a welding power supply during a welding process.

SUMMARY OF THE INVENTION

The object of further improving a welding process is achieved by amethod of operating a welding power supply during a welding processaccording to claim 1.

According to the inventive method an electric arc between a consumableelectrode and a work piece is generated while feeding a consumableelectrode and moving the arc in relation to the work piece along awelding track. During the welding process a transition is made between aDC power output of the welding power supply and an AC power output ofthe welding power supply, or vice versa. Hence according to theinvention the transition between a DC power output and an AC poweroutput, or vice versa, is made without interruption of the weldingprocess.

By, as is proposed by the inventive method, allowing a transitionbetween AC and DC output during a welding process while feeding aconsumable electrode and moving the arc in relation to the work piecealong a welding track it is possible to adapt the weld process torapidly changing welding conditions such as a transition between a rootrun and a following hot pass. When performing a root run a deeppenetration is required in order to make a fully dense joint between twoopposing end portions facing each other with a narrow gap in between. Byoperating the power supply to provide a DC-positive output deeppenetration is ascertained. When the root run is completed it is oftendesirable to complete the weld seam by providing hot pass followed byone or more fill passes. In such circumstances the invention proposes toperform a shift to an AC process for a provision of one or more fillerruns from a DC-positive electrode process completing the root runwithout interrupting the welding process. This means that the feeding ofthe consumable electrode as well as movement of the arc in relation tothe work piece will continue at the transition. By avoiding interruptingthe process it can be assured that the weld puddle is not solidified orcooled. Hence, fusion defects can be avoided at the location of thetransition between the end of the root run and the beginning of thefiller strings. According to prior art methods, chamfering may be neededin order to avoid possible fusion defects at the location of the startof the AC process following after the root run. By using the methodaccording to the invention a time consuming chamfering process is beavoided.

A further advantage of the inventive process is that effective arc blowprevention may be performed at welding tracks having complex geometry,where arc blow occurs at an unacceptable level at certain segments ofthe weld track, while the arc blow is on a low level at other segmentsof the weld track. In such situations, a transition between a DC poweroutput of the welding power supply and an AC power output of the weldingpower supply or vice versa may be made in dependence of the specific arcblow condition at the location. A DC power output may be used in thesegments where the arc blow is low due to geometry and a transition toan AC process can be made without interruptions at segments where thegeometry induces large arc blow. By allowing the transition to takeplace, the benefits of the DC process can be used for certain segmentswhile a reduction of the arc blow due to the use of an AC process forother segments is allowed without interruption of the welding process.

It is therefore contemplated to optionally perform an assessment of aparameter representing arc blow at a welding location and to adjust abalance in dependence of the assessed parameter value.

At the transition between the AC and DC processes, the welding speed andthe electrode feed speed may be maintained. This means thatv_(DC)(t0)=v_(AC)(t0), where v_(DC)(t0) is the welding speed of the DCprocess at the time of transition t0 and v_(AC)(t0) is the welding speedof the AC process at the time of transition. Similarly,w_(DC)(t0)=w_(AC)(t0), where v_(DC)(t0) is the electrode feed speed ofthe DC process at the time of transition t0 and v_(AC)(t0) is theelectrode feed speed of the AC process at the time of transition.Optionally, the electrode feed speed and the welding speed at steadystate may be different from the electrode feed speed respectively thewelding speed at the transition. This means that v_(DC)(ts)≠v_(DC)(t0)for ts≠t0, where ts are the time at which the process is run at steadystate. Further, v_(AC)(ts)≠v_(AC)(t0) for ts≠t0, w_(DC)(ts)≠w_(DC)(t0)and w_(AC)(ts)≠w_(AC)(t0). The transition between the AC and DCprocesses can be smoothened by allowing ramps of the electrode feedspeed at the transition. In the event the location of a discontinuity atwhich the process should change from AC to DC is known prior to arrivingat the discontinuity, the ramp can be distributed on both sides of thetransition

In order to generate a suitable weld an AC balance value may be set inorder to provide an appropriate penetration value for a weld process tobe performed. The AC balance is a ratio between the electrode positiveand electrode negative. The AC balance B is defined as proportion ofelectrode positive during a weld cycle. A balance of 100% DC positivehas no DC negative component in a weld cycle. A balance of 0% DCpositive has no DC positive component in a weld cycle. In an embodimentof the invention, a balance between positive electrode potential andnegative electrode potential during a welding cycle is thus continuouslyadjustable between DC-negative electrode and DC-positive electrode viathe AC power output. By allowing a continuously adjustable balancebetween 0 and 100% DC-positive electrode at an AC process, the characterof the arc can be suitable adapted to the welding conditions.

In one embodiment of the invention, it is therefore suggested to assessa surface profile of the weld bead at a welding location and adjust thebalance in dependence of the surface profile at the welding location.The assessment may be based on a predefined map including information ofa surface profile as a function of the welding location. The surfaceprofile is the geometry of the weld track at the welding location. Inthe assessment, a desired welding penetration profile may be retrievedfor the welding location where after the balance may be set independence of the desired welding penetration profile at the location.

The assessment may include determining of a current welding location andretrieving a value representing the desired balance at the currentwelding location.

Optionally a sensor may be used to determine a surface profile of thewelding track at the welding location and the balance may be set inaccordance with the detected surface profile.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will be described in further detail belowwith reference to appended drawings, where:

FIG. 1 shows a schematic drawing of a welding arrangement according tothe invention,

FIG. 2 shows a schematic drawing of a pipe welding process,

FIG. 3 shows a schematic drawing of a weld seam including a root portionat an area where a transition from an uncompleted root run to acompleted root run is located,

FIG. 3a shows a symbolic drawing of a weld track having a surfaceprofile with a wedge portion and a root portion,

FIG. 4 shows a diagram of a weld depth as a function of the positionalong a weld track,

FIG. 5 shows a diagram with a desired balance as a function of thewelding location,

FIG. 6 shows a schematic map of a welding process along a track, and

FIG. 7 shows a schematic flowchart of an embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

FIG. 1 shows a welding arrangement 10 suitable for use in a methodaccording to the invention. The welding arrangement includes a weldingpower source 20 capable of operating in DC mode as well as AC mode. Thewelding power supply is suitably an inverter power source which may beof a design as presented in U.S. Pat. No. 5,710,696. The power sourceincludes a DC stage 30 connected to an AC input 31 and a DC output 32.The DC stage 30 includes a transformer stage 33 which generates a lowvoltage high current output to a rectifier stage 34, which may bedesigned by a diode bridge and a capacitor. The output from therectifier stage is provided to a switching regulator 35 includingswitches which provides a chopped DC output signal of the output fromthe rectifier circuit. The chopping frequency is generally around 10-25kHz.

The output from the switching regulator 35 is fed to an AC stage 40formed by an inverter circuit 41. The inverter circuit 41 includes a setof switches 42 enabling transformation from the DC input to an AC outputin a conventional manner as disclosed in U.S. Pat. No. 5,710,696. Whenoperating in DC mode the inverter circuit 41 is controlled to provide aconstant polarity DC output. This is performed by not switching betweenthe states of the switches 42. When operating in DC mode, either a DCnegative output or a DC positive output may be provided depending on theselected control of the switches 42.

A control arrangement 50 is arranged to control the output of thewelding power supply 20. The control arrangement includes a digitalsignal processor 51. The digital signal processor 51 serves to controlthe shape of current pulses by controlling the switches of the switchingregulator 35. For this purpose the digital signal processor 51 mayinclude a pulse width modulator under control of a wave shaper locatedin a general controller 54. The control of the switches may be performedin a manner as disclosed in U.S. Pat. No. 5,715,150.

The digital signal processor 51 receives as input signals current andvoltage output values detected by sensors 52, 53 at the output from theinverter 41 or from the switching regulator 35. Furthermore the generalcontroller 54 determines desired values of the welding voltage V,welding current I, electrode feed speed, w, and welding speed v. Thesevalues may be set by an operator from an operator interface 55, or froma map 56 containing preset welding parameters depending on selected weldcases.

The digital signal processor 51 is furthermore responsible forcontrolling the balance of an AC output from the inverter circuit 41, ifprovided. This control is performed by setting switching times of theswitches 42.

Optionally the controller receives an input from a welding profilesensor 61, which determines the profile of a surface at a weldinglocation. The welding profile sensor may determine a desired weldpenetration that is a welding depth at the welding location. The weldingdepth may be defined as a distance between a highest and lowest pointwithin a welding area, where the welding area corresponds to a weldpuddle at the welding location. Since welding may be performed with awork piece inclined at any desired angle with respect to the verticalplane, the depth is measured in the direction between the arc and thepoint of the weld puddle having the deepest penetration. In the case ofa weld performed in the vertical plane, the depth will be measured inthe vertical plane. In the root run the depth will correspond to thethickness of the root, while at the filler rounds the depth will dependon the weld profile.

Instead or complementary to the weld profile sensor 61 a weld profilemap 62 may be provided. The weld profile map includes data representinga desired welding depth or a desired balance as a function of thewelding location or a combination thereof. Expressed in weldingcoordinates s, where s is a location along a welding track the map maybe expressed as B(s) where B is the balance at a desired locationproviding a desired welding penetration D at a location s along awelding track. Alternatively the map may be expressed as D(s), whereD(s) is a desired penetration at a location s along a welding track. Inthis event, a map between a desired balance and a desired weldingpenetration profile should be provided. The map 63 between a desiredwelding penetration profile D and a desired balance value may be storedin a memory area 64 accessible for the general controller 54. The map 63is created from experimental results from different weld cases.Optionally a ramp controller 65 is connected to the general controller54. The ramp controller controls the process parameters welding voltageV, welding current I, electrode feed speed, w, and welding speed v at adetected discontinuity along the welding track.

The welding arrangement further includes a welding robot 70 including atleast one welding head 71 through which a consumable electrode 72 isfed. The welding robot 71 further includes an electrode feeder 73arranged to feed the welding consumable electrode 72 at a desiredelectrode feed speed w. The welding arrangement includes a propulsionunit arranged to generate a relative movement between the work piece andthe welding head. The propulsion unit may be provided by a movablewelding robot, which may propagate in relation to the work piece or byarranging the work piece to be movable. In FIG. 1, the welding robot 70is movable, while in FIG. 2, which shows an arrangement for pipe weldingincluding a pipe support 81 on which a pipe 82 is located. A fixedwelding head 83 is arranged to provide an arc at a specified location.The pipe support 81 includes one or more driven rollers which rotatesthe pipe.

FIG. 3 shows a schematic drawing of a weld seam 90 including a rootportion 91 and a wedge portion 92 of a work piece 102. The root portionshould be welded in a root run providing a complete root weld. Thewelding is preformed along a welding track 103. A welding head 93 guidesa consumable electrode 94 at which an arc 95 is formed. The schematicdrawing shows an area 96 where a transition 97 from an uncompleted rootrun 99 to a completed root run 98 is located. The welding is performedin the direction indicated by the arrow 100. At a current weldinglocation S(t) welding is performed at an open root. At the locationS(t₀₎ a transition is made to a location where a completed root ispresent. Welding should here be continued in the wedge portion bysupplying filler strings.

In FIG. 3a a symbolic drawing of a surface profile 301 having with awedge portion 92 and a root portion is shown. The surface profile maycontain information about both the wedge portion, which is to be weldedin a later subsequent or subsequent runs, and the root portion which isto be welded in an initial run. That is the surface profile 31 maycontain information relating to the ongoing and coming welding runs.Alternatively the surface profile only contains information relating tothe coming run. The surface profile may contain information relating tothe depth and width of the gap which is to be joined. A complete surfaceprofile as exemplified in FIG. 3a may contain information regarding thedepth of the root d_(r), the width of the root w_(r), the wedge portiond_(w), and the width of the wedge portion w_(w). In FIG. 3a a rootstring 303 formed during a root pass and filler strings 305, 307, 309formed during subsequent passes are shown.

When performing a root run a deep penetration is required in order tomake a fully dense joint between two opposing end portions facing eachother with a narrow gap in between. By operating the power supply toprovide a DC-positive output deep penetration is ascertained. When theroot run is completed it is often desirable to complete the weld seam byproviding one or more filler strings on top of the root run. In suchcircumstances the invention proposes to perform a shift to an AC processfor a provision of one or more filler runs from a DC-positive electrodeprocess completing the root run without interrupting the weldingprocess. This means that the feeding of the consumable electrode as wellas movement of the arc in relation to the work piece will continue atthe transition. By avoiding interrupting the process it can be assuredthat the weld puddle 101 is not solidified or cooled. Hence, fusiondefects can be avoided at the location of the transition between the endof the root run and the beginning of the filler strings. According toprior art methods, chamfering may be needed in order to avoid possiblefusion defects at the location of the start of the AC process followingafter the root run. By using the method according to the invention atime consuming chamfering process may be avoided.

At the transition between the DC and AC processes, the welding speed andthe electrode feed speed is maintained. The transition takes place atthe location to where the discontinuity from the root run to the fillerrun is located. This means that v_(DC)(t0)=v_(AC)(t0), where v_(DC)(t0)is the welding speed of the DC process at the time of transition t0 andv_(AC)(t0) is the welding speed of the AC process at the time oftransition. Similarly, w_(DC)(t0)=w_(AC)(t0), where v_(DC)(t0) is theelectrode feed speed of the DC process at the time of transition t0 andv_(AC)(t0) is the electrode feed speed of the AC process at the time oftransition. Optionally, the electrode feed speed and the welding speedat steady state may be different from the electrode feed speedrespectively the welding speed at the transition. This means thatv_(DC)(ts)≠v_(DC)(t0) for ts≠t0, where ts are the time at which theprocess is run at steady state. Further, v_(AC)(ts)≠v_(AC)(t0) forts≠t0, w_(DC)(ts)≠w_(DC)(t0) and w_(AC)(ts)≠w_(AC)(t0). The transitionbetween the AC and DC processes can be smoothened by allowing ramps ofthe electrode feed speed at the transition. In the event the location ofa discontinuity at which the process should change from AC to DC isknown prior to arriving at the discontinuity, the ramp can bedistributed on both sides of the transition.

In FIG. 4 a desired welding penetration profile as a function of alocation along a welding track is disclosed. At the location t₀ where adiscontinuity between a root run and a filler run is present. Thedesired welding penetration profile D falls at this location from a highvalue corresponding to a DC-positive output power mode to a lower valuecorresponding to an AC output having a certain balance B.

A corresponding diagram with a desired balance B as a function of thewelding location is disclosed in FIG. 5. The transition 503 from theDC-positive output power mode 501 to the AC output power mode 502 maycontain a ramp continuing over several AC pulses or be performed in asingle pulse, where the process is changed from a steady stateDC-positive process to a steady state AC process having a certainbalance value via a transition phase with a change of the balance valueover the several pulses. This is indicated by the smooth portion betweenthe DC-positive process and the steady state AC process occurring aftert₀+Δ.

Effective arc blow prevention may be performed at welding tracks havingcomplex geometry, where arc blow occurs at an unacceptable level atcertain segments of the weld track, while the arc blow is acceptable atother segments of the weld track. Here a transition between a DC poweroutput of the welding power supply and an AC power output of the weldingpower supply, or vice versa, in dependence of the specific arc blowcondition at the location. A DC power output may be used in the segmentswhere the arc blow is low due to geometry and a transition to an ACprocess can be made without interruptions at segments where the geometryinduces large arc blow. By allowing the transition to take place, thebenefits of the DC process can be used for certain segments while areduction of the arc blow due to the use of an AC process for othersegments is allowed without interruption of the welding process.

It is therefore contemplated to optionally perform an assessment of aparameter representing arc blow at a welding location and to adjust abalance in dependence of the assessed parameter value. This may bepossible by either detecting an arc blow condition and changing theoutput power from DC to AC where arc blow is detected or by providing amap over the desired welding condition as a function of the positionalong a welding track, where at locations sensitive to arc blow atransition from DC output to AC output is made without interrupting thewelding process.

In FIG. 6 a schematic map of a welding process along a track definedalong a coordinate S is indicated. The map includes two areas A1 and A2where a DC-positive process is run and three areas A3, A4, A5 withdifferent balance values are performed.

In FIG. 7 a schematic flowchart of an embodiment of the invention isshown. In a first process step S10 an AC or DC output power process isrun at a electrode feed speed w and a welding speed v. At a step S20 itis determined whether a discontinuity is present, such as a transitionfrom a root run to a filler run or from an acceptable arc blow conditionto an unacceptable arc blow condition takes place. The discontinuity canbe detected by sensors or by a map indicating a discontinuity. If adiscontinuity is present the process will in a step 30 change from DC toAC or vice versa depending on the nature of the discontinuity. If adiscontinuity is not present, the process will return to step 10 if thecurrent process is a DC process. If the current process is an AC processthe balance value may be determined in a process step S50. The desiredbalance value may be obtained from a map 62.

The welding process may include a root pass and one or more fill passes,wherein said root pass is made with a DC-positive electrode. The fillpasses are made with an AC power output, and that a transition betweenthe DC-positive electrode and the AC power output is made withoutinterruption of the welding process.

The welding process may thus include the steps of performing a root passwith a DC-positive electrode, determining that the root pass iscompleted, and switching to an AC power output when said root pass iscompleted without interrupting the welding process.

The process allows for an adjustable balance between negative electrodepotential and positive electrode potential during a welding cycle, whichbalance may be continuously adjustable between DC-negative electrode andDC-positive electrode via the AC power output.

In a step S60 a surface profile of the welding track at a weldinglocation is assessed and that the balance is adjusted in dependence ofthe surface profile at the welding location. In one embodiment theassessment is based on retrieving a desired welding penetration profileat the welding location and setting the balance in dependence of thedesired welding penetration profile at the location. Optionally theassessment is based on a predefined map including information of thesurface profile as a function of the welding location.

Optionally assessment includes determining of a current welding locationand retrieving a value representing the desired balance at the currentwelding location. Optionally a sensor determines a surface profile ofthe welding track at the welding location.

The invention claimed is:
 1. A method of operating a welding powersupply during a welding process in which an electric arc between aconsumable electrode and a work piece is generated while feeding theconsumable electrode and moving the arc in relation to the work piecealong a welding track, wherein the method comprises: receiving profileinformation, including at least a weld profile map, the weld profile mapincluding data representing a desired welding depth, a desired balanceas a function of the welding location, or a combination thereof, whereinthe desired balance comprises a ratio between electrode positive andelectrode negative; and based upon the profile information, performing afirst transition between a DC positive output power mode of the weldingpower supply and an AC output power mode of the welding power supply,and performing a second transition between the AC output power mode ofthe welding power and the DC positive output power mode of the weldingpower, wherein the first transition and the second transition are madewithout interruption of the welding process.
 2. The method of operatinga welding power supply according to claim 1, wherein a balance between apositive electrode potential and a negative electrode potential during awelding cycle is continuously adjustable between a DC-negative electrodeand a DC-positive electrode via the AC power output.
 3. The method ofoperating a welding power supply according to claim 1, wherein thereceiving the profile information comprises determining a surfaceprofile of the welding track at a welding location, and wherein abalance between a positive electrode potential and a negative electrodepotential during a welding cycle is adjusted depending on the surfaceprofile at the welding location.
 4. The method of operating a weldingpower supply according to claim 3, wherein the receiving the profileinformation comprises retrieving a desired weld penetration profile atthe welding location, and wherein the balance is set depending on thedesired welding penetration profile at the location.
 5. The method ofoperating a welding power supply according to claim 4, wherein the weldprofile map comprises a predefined map including information of thesurface profile as a function of the welding location.
 6. The method ofoperating a welding power supply according to claim 5, wherein thereceiving the profile information further includes determining a currentwelding location and retrieving a value representing a desired balancebetween a positive electrode potential and a negative electrodepotential during a welding cycle at the current welding location.
 7. Themethod of operating a welding power supply according to claim 4, whereinthe receiving profile information further comprises receivinginformation from a sensor, wherein the sensor determines a surfaceprofile of the welding track at the welding location.
 8. The method ofoperating a welding power supply according to claim 1, wherein saidwelding process includes a root pass forming a root string and one ormore fill passes forming filler strings, wherein the root pass is madewith a DC-positive electrode, the fill passes are made with an AC poweroutput, and a transition between the DC-positive electrode and the ACpower output is made without interruption of the welding process.
 9. Themethod according to claim 1 including the steps of performing a rootpass with a DC-positive electrode, determining that the root pass iscompleted, and switching to an AC power output when the root pass isdetermined to be completed without interrupting the welding process. 10.The method according to claim 1 including assessing a parameterrepresenting an arc blow at a welding location and adjusting a balancedepending upon a value of the assessed parameter.